Connecting to a Database

Connecting to a Database

Express is database-agnostic. You can connect to any database by installing the appropriate driver or ORM. The most common choices in the Node.js ecosystem are Prisma (type-safe ORM), Drizzle (lightweight ORM), Mongoose (MongoDB ODM), and raw pg for PostgreSQL.

Pattern: Create a Singleton Client

Create a single database client instance and export it. Importing it in multiple route files reuses the same connection pool rather than opening a new connection per request.

// src/lib/db.ts (Prisma example)
import { PrismaClient } from "@prisma/client";

declare global {
  // eslint-disable-next-line no-var
  var __prisma: PrismaClient | undefined;
}

export const db = globalThis.__prisma ?? new PrismaClient();

if (process.env.NODE_ENV !== "production") {
  globalThis.__prisma = db;
}

The global assignment prevents hot-reload from opening a new Prisma client on every file change during development, which would exhaust the connection pool.